1. Field of the Invention
The invention relates to the selectively variable transmission of rotational motion and more particularly to a variable transmission arrangement wherein sprags transmit torque between eccentrically journalled input and output members whereby control of the degree of eccentricity determines the drive ratio.
2. Description of the Prior Art
One type of overrunning clutch employs a plurality of sprags interposed between a pair of concentrically mounted rotatable members to transmit rotational motion in only one direction from a driving to a driven member. The sprags used are so positioned and have their friction surfaces so contoured as to divide the load as equally as possible among the several sprags of the clutch and to assure sufficient tangential friction on each sprag to meet torque requirements. Because of the use of a jamming drive angle in sprag clutches, slipping is eliminated and the drive forces are proportional to the output torque, so that the clutch is heavily loaded only when demand is heavy. The transmission of the present invention has certain features, such as the use of a jamming angle, in common with sprag clutches.
Because the present invention is exemplified by an embodiment particularly suited for use as a bicycle transmission, it should be noted that a wide variety of bicycle transmission systems have been proposed, and a number of different systems are found on modern bicycles. Two speed, three speed and five speed bicycles are available that have transmissions built into their rear hubs. Another popular type of bicycle transmission is the "derailleur", a chain drive in which the chain is shifted from one to another of a plurality of coaxially mounted sprockets to change the drive ratio. A disadvantage of the derailleur system is that the transmission should not be loaded while shifting from one sprocket to another. Derailleur systems are typically quite sensitive and demand frequent repair and adjustment.